Disclosed is a method of tightly packing display elements in an electric display. The method involves increasing the density of display elements of an electronic display, such as rotatable elements in electronic paper, in order to increase the brightness and/or sharpness of the display.
Traditional signs have been based upon printed materials, paper, plastic, metal, etc., and are therefore not programmable. Accordingly, they are not easily changed. In an attempt to overcome this problem, electronically programmable and/or controllable signs have been in existence for many years. For example, liquid crystal diode (LCD) displays, cathode ray tube (CRT) displays, and other electrically-addressable displays will display an image in response to applied electric signals or fields. However, such signs typically require a large amount of electricity, since they must provide illumination in order to be visible to a viewer.
Various types of electric writeable media, some of which are commonly known as rotatable element displays or electric paper displays, also exist in the prior art. One example of a rotatable element display includes a polymer substrate and bichromal rotatable elements such as balls or cylinders that are in suspension with an enabling fluid and-are one color, such as white, on one side and a different color, such as black, on the other. Examples of such rotatable element displays are described in U.S. Pat. No. 5,723,204 to Stefik and U.S. Pat. No. 5,604,027 to Sheridon, each of which is incorporated herein by reference in its entirety. Under the influence of an electric field, the elements rotate so that either the white side or the black side is exposed.
Another type of electric writeable media is known as an electronic ink display, such as the one described in U.S. Pat. No. 6,518,949 to Drzaic, which is incorporated herein by reference. An electronic ink display includes at least one capsule filled with a plurality of particles, made of a material such as titania, and a dyed suspending fluid. When a direct-current electric field of an appropriate polarity is applied across the capsule, the particles move to a viewed surface of the display and scatter light. When the applied electric field is reversed, the particles move to the rear surface of the display and the viewed surface of the display then appears dark.
Yet another type of electric writeable media, also described in U.S. Pat. No. 6,518,949 to Drzaic, includes a first set of particles and a second set of particles in a capsule. The first set of particles and the second set of particles have contrasting optical properties, such as contrasting colors, and can have, for example, differing electrophoretic properties. The capsule also contains a substantially clear fluid. The capsule has electrodes disposed adjacent to it connected to a voltage source, which may provide an alternating-current field or a direct-current field to the capsule. Upon application of an electric field across the electrodes, the first set of particles move toward one electrode, while the second set of particles move toward the second electrode.
Rotatable element displays have numerous advantages over conventional displays, such as LCDs and CRTs, since they are suitable for viewing in ambient light, they retain an image indefinitely in the absence of an applied electric field, and they can be made to be very lightweight and/or flexible. For further advantages of such displays, see U.S. Pat. No. 5,389,945 to Sheridon, incorporated herein by reference in its entirety. An example of such a display is a SmartPaper™ display from Gyricon LLC.
One disadvantage of prior art electric paper displays is that they are generally not as bright as LCD or CRT displays. This is at least in part because the current techniques used to place changing elements in electric paper displays do not pack the elements sufficiently close to adjacent elements. When changing elements are placed in an electric paper display, electric charges placed on the elements tend to repel the elements from adjacent elements. Tight element packing has been shown to be necessary for high brightness of an electric paper display.
A slurry coating technique is used to place elements in electric paper. The technique starts by dispersing changing elements, such as bichromal beads, in an uncured elastomer to form a slurry. The slurry, which is approximately 50% bichromal beads and 50% elastomer by weight, is then applied to a release substrate with the use of an applicator known as a “doctor blade.” Heat is applied to the elastomer/bichromal bead coating in order to cure the coating. Subsequently, the heated slurry is pulled from the surface of the release substrate.
This technique has substantial limitations relating to the viscosity of the slurry. In order to make slurry coatings, the viscosity of the slurry must be relatively low. However, the viscosity of the slurry is largely determined by the percentage of the slurry composed of bichromal beads. As such, the concentration of bichromal beads in the slurry is not high enough to guarantee tight bead packing in the exterior layers of the electric paper sheet.
As such, a need exists to improve electric paper by creating methods of more closely packing rotatable elements in electric paper to improve the brightness of the paper and to allow for easier viewing.